"Russia launched a satellite into low Earth orbit that we assess is likely a counter space weapon," said Pentagon spokesman Brig Gen Pat Ryder on Tuesday evening.
It was on the "same orbit" as a US government satellite, he said, adding that Washington would continue to monitor the situation and had to be ready to protect its interests.
Russia has not publicly commented on the issue.
On Tuesday, Gen Ryder said the Pentagon believed the Russian satellite was "presumably capable of attacking other satellites in low Earth orbit".
"Russia deployed this new counter space weapon into the same orbit as a US government satellite.
"And so assessments further indicate characteristics resembling previously deployed counter space payloads, from 2019 and 2022.
"We have a responsibility to be ready to protect and defend, the domain, the space domain, and ensure continuous and uninterrupted support to the Joint and Combined Force," the Pentagon spokesman added.
Moscow and Washington - two global rivals - have repeatedly clashed over space weapons issue at the UN in recent weeks, with both sides accusing each other of seeking to militarise space.
Earlier on Tuesday Russia's foreign ministry spokeswoman Maria Zakharova claimed the US was seeking to turn space into an "arena for military confrontation".
A number of military experts have long warned that space is likely to be the next frontier of warfare in an increasingly technology-dependent world.
In February, the White House admitted Russia was developing a "troubling" new space weapon, but insisted that it had yet to deploy it.
It came after a senior Republican congressman issued a cryptic warning about a serious national security threat, sparking intense rumours around the Washington DC.
A report released by the Washington DC-based Center for Strategic and International Studies last year suggested that Russia is developing a range of anti-satellite (ASAT) weapons, including a missile that was successfully tested against a defunct Soviet-era satellite in November 2021.
Separately, a spokesperson for the US Space Command told Reuters news agency that the satellite was "likely a counter space weapon presumably capable of attacking other satellites in low Earth orbit".
The spokesperson said the satellite - Cosmos 2576 - was launched on 16 May from Russia's Plesetsk cosmodrome, about 800km (497 miles) north of Moscow.
In its statement, Russia's Roskosmos state space agency said the launch on 17 May was "in the interests of the defence ministry of the Russian Federation". It said its launch vehicle Soyuz-2.1b was used.
The different reported launch dates might be explained by the fact that the Moscow time zone is three hours ahead of GMT.
Neither Moscow nor Washington provided any further details.
But space analysts say that Cosmos 2576 appears to be on the same orbit as America's USA 314 satellite.
Since launching a full-scale invasion of Ukraine in February 2022, Russia has warned that US satellites aiding the Ukrainian military could become legitimate targets.